Abstract

The Ombilin Basin is a Tertiary intermontane basin located within the Barisan Mountain Range of West Sumatera. This basin was totally unexplored for oil and gas until P.T. Caltex Pacific Indonesia initiated exploration activities in February 1981 after signing the Singkarak Block Production Sharing Contract with Pertamina. Geological field mapping, an airborne radar (SAR) survey, and gravity and seismic surveys were carried out in the Ombilin Basin and provided a wealth of new geological and geophysical information which led to the drilling of the Sinamar No. 1 exploratory well to a total depth of 9902 ft (3018 m).

Sinamar No. 1 is a significant well in the history of Indonesia's oil industry since it is the first oil and gas exploration well drilled in an intermontane basin in Indonesia. In addition to being the first well drilled in the Ombilin Basin, Sinamar No. 1 also represents the first oil and gas exploratory drilling in the Province of West Sumatra.

Seismic and well data indicate that despite the small areal size of the Ombilin Basin (approximately 1500 sq km), up to 15,000 ft (4600 m) of Tertiary sediments are present. The Tertiary section in the basin ranges in age from Middle Eocene to Early Miocene. Significant depositional hiatuses occur within the Tertiary succession. Previous geological mapping by the Institute of Technology Bandung and information obtained by the recent oil and gas exploration indicates that the Ombilin Basin was a true intermontane basin during its Early Tertiary history. Massive debris flows and extensive alluvial fan deposits occur on the basin margins and in a large Eocene lake occupied the basin center. Uplift and erosion since Middle Miocene has reduced the Ombilin Basin to its present areal configuration. Although this basin is located within Sumatra's magmatic arc and is partially covered by volcanics from extinct and active volcanoes, subsurface temperature gradients are significantly cooler than those in the Sumatra back-arc basins. Eocene lacustrine shales and Oligocene marine shales are the likely source rocks for the hydrocarbons tested in Sinamar No. 1 and the oil seepages located along the basin margins.